I have to go with a series that is only possible to play if you own the original consoles and games (one that has never had a port). And for those who know me, are you honestly surprised that it's Star Wars?!
The Rogue Squadron trilogy is amazing (particularly Rogue Leader) and deserves to be played by more people! And Battle for Naboo would be a nice added bonus (same developer and similar gameplay).
Sadly, a remaster for the Wii was completed but never released (and Factor5 has since gone bankrupt). But LucasArts owns the rights, so this is certainly within the realm of possibility!
I would love to see the entire series be co-op (perhaps even up to 4 players) with updated graphics and controls!
Edit: I’m glad to see so many people agree with me on this one (so far the only game with multiple votes)! Now if only LucasArts would realize the gold mine they’re sitting on and give us this compilation!
Honourable mention to the answer
@dennis2130 gave - it'd be great to have the DK games all together in one collection. Though I'd settle for DKC 1-3, DK64, and Diddy Kong Racing in one package!